This invention relates to an X-ray generator using tetrode tubes as switching elements for an X-ray tube of a center metal-grounded type.
In the X-ray generator of this type, a high-tension power source is connected to the anode and cathode of an X-ray tube respectively, through switching elements (for example, tetrode tubes), with the center metal electrode of the X-ray tube grounded, and X-rays are generated by applying a high voltage across the anode and center metal, and across the cathode and center metal.
When, in this arrangement, a high voltage is supplied to the cathode, not the anode, due to, for example, a breakage of a cable on the anode side while the filament of the X-ray tube is heated, there is a risk that the X-ray tube will be broken, due to an abnormal current flowing between the cathode and center metal electrode of the X-ray tube.
Japanese patent laid-open publication No. 56-94800 discloses the concept of detecting an abnormal current in the cathode, to stop the projection of X-rays. According to this prior art, if an abnormal current flows due to a switch, on the cathode side of an X-ray tube, being not opened on account of a short-circuiting fault, this is detected by an abnormal-current detection circuit, in which case an interlock circuit issues an X-ray-stopping instruction to an X-ray control circuit, in order to protect the X-ray tube. That is, the conventional protection means functions to cut a high-tension power source and heater power source of the X-ray tube and thus to cease the supply of a voltage to the X-ray tube. As such a high-tension power source, a type is known which includes rectifying circuits. In such rectifying circuits for converting an AC voltage supplied from an AC power source, through a high-tension transformer, to high DC voltages, smoothing capacitors following rectifiers are required. To prevent X-ray projection or irradiation from the X-ray tube, in the event of a short-circuiting fault of a cathode-side switch, a high-tension switch on the anode side, as well as a switch on the AC power source side, is opened subsequent to the detection of an abnormal current. Since, however, the smoothing capacitors are already charged by the AC power source, a current flows from the capacitor, through the switch, into a cathode-to-center metal electrode path of the X-ray tube. Since a current flowing through the center metal electrode at an abnormal time is much greater than a current flowing at a normal X-ray projection time, there is a risk that the X-ray tube will be broken, due to the fusion of the center metal electrode.